This invention relates generally to combined cycle power systems, such as those used in a power plant, and more specifically, to methods and systems for completing a hot purge of a heat recovery steam generator (HRSG).
At least some known combined cycle power systems include at least one gas turbine which powers a generator used to supply electrical power to a power grid. Exhaust from at least some known gas turbines is supplied to an HRSG that extracts heat from the exhaust to generate steam for use in other processes such as, but not limited to, driving a steam turbine and/or providing hot steam for heating or desalinization. Power generated by a steam turbine also drives an electrical generator that provides additional electrical power to the power grid.
Combined cycle power systems, that include a bypass damper in the exhaust flowpath, are often designed such that the bypass damper can be used to facilitate transitioning a gas turbine's exhaust path from simple cycle to combined cycle, and back, while under load. The primary regulatory guidance governing this equipment is NFPA 85 and ISO 21789. The practice of transitioning between modes is expressly permitted if the system startup includes a cold purge of both the exhaust flow path through the gas turbine and the HRSG.
However, more system operators are demanding operating scenarios in which a post-operational purge is conducted before placing equipment in the exhaust path into service, such as an HRSG. If a transition from simple cycle to combined cycle operation is required, it may be desirable to do so without shutting down the gas turbine. Such a practice, known as “hot purging” an exhaust path, is recognized in NFPA 85, but with strict limits. For example, the operation may only occur when the gas turbine exhaust temperatures are at least 100° F. below the lowest auto-ignition temperature (AIT) of any fuels or fuel mixes that may be introduced into either the gas turbine or any downstream firing equipment.
In addition, the International Organization for Standardization (ISO) has developed ISO 21789, which is a worldwide gas turbine safety standard. This standard permits gas turbine exhaust gases to be used for purging provided that they are proven, and controlled, to be less than 80% of the AIT of any flammable gases or vapors that may be present.